High-voltage (“HV”) transistors are broadly used in integrated high-voltage power management circuits for industrial and consumer electronic devices. For a high-voltage transistor used as a power transistor in such high-voltage power management circuits, of which the input voltage may be as high as 1000V, it is desired that the high-voltage transistor have a high breakdown voltage (i.e. be capable of withstanding high-voltage without damage) to enhance the operational stability of the power management circuits. It is also desired that the high-voltage transistor have a low on-resistance to improve the efficiency of the power management circuits.
Usually, the on-resistance of the high-voltage transistor can be decreased by increasing the doping concentration within a drift region between a drain region and a source region of the high-voltage transistor. However, increasing the doping concentration within the drift region make it more difficult for the drift region to be completely depleted, resulting in a decrease in breakdown voltage. Therefore, it is desired to provide a high-voltage transistor device having low on-resistance without decreasing the breakdown voltage.
Moreover, a high-voltage transistor in the high-voltage power management circuits generally operates with ON/OFF transitions, during which the high-voltage transistor may experience very large and fast drain-to-source voltage changes. For example, when a high-voltage MOS transistor is switched from an ON state to an OFF state, the drain voltage of the MOS transistor may increase rapidly from a relatively low voltage (e.g., 0V-20V) to a quite high voltage (e.g., more than 400V) in a very short time (e.g. less than 1 microsecond), which may cause the high-voltage MOS transistor to break down while it has not yet established the ability to withstand a high voltage as in steady state. Thus, a high-voltage transistor should withstand such large and fast voltage changes without damage during ON/OFF transitions, i.e. the high-voltage transistor should have a high dynamic breakdown voltage.